The present invention relates to compositions and methods for the stripping of tin, lead or tin-lead alloys (i.e., solder) from a copper metal surface, and more particularly to compositions and methods for treating tin-coated or lead-coated or solder-coated copper surfaces in the manufacture of printed circuit boards to strip the tin or lead or solder therefrom and expose the underlying copper metal.
In the course of manufacturing printed circuit boards, it is commonplace to deposit (e.g., by electroplating, immersion or other like processes) a layer of tin, lead or tin-lead alloy (solder) on all or selected conductive copper surfaces of the board defining traces, through-holes, surrounding pad areas and the like, to serve, for example, as an etch resist in the subsequent etching away of other copper surfaces. By the same token, it is necessary to eventually strip the tin or lead or tin-lead alloy from all or selected copper surfaces coated therewith, as is needed for example when it is desired to place certain copper surfaces (e.g., contact fingers) with nickel and/or gold to improve conductivity, or when it is desired to apply a solder mask over bare copper surfaces (SMOBC processes), or when it may be necessary simply to treat a reject piece in an effort to recover and re-use the underlying copper material. Also, while particularly apropos of printed circuit board manufacture, the need to strip away tin, lead or tin-lead layers from copper surfaces also arises in other contexts where tin, lead or tin-lead has been applied over a copper surface for decorative and/or functional purposes.
In addition, when copper metal surfaces are coated with tin or tin-lead alloys, a thin layer or film of copper-tin alloy (or intermetallic) typically forms at the layer interface, which film progressively increases in thickness with time. Accordingly, in processes designed to strip away the tin or tin-lead layer to expose copper metal, it is necessary to insure that this copper-tin intermetallic also is removed.
Compositions designed to strip tin and/or tin-lead coatings from copper surfaces are known in the art. One class of such compositions includes those based upon hydrogen peroxide and hydrofluoric acid or a fluoride. See, e.g., U.S. Pat. Nos. 3,926,699; 3,990,982; 4,297,257; 4,306,933; 4,374,744 and U.S. Pat. No. 4,673,521. Another class involves those employing nitro-substituted aromatic compounds as a principal ingredient, often in conjunction with an inorganic acid (see, e.g., U.S. Pat. Nos. 3,677,949; 4,004,956; and U.S. Pat. No. 4,397,753) or an organic acid (see U.S. Pat. No. 4,439,338 disclosing the use of alkylsulfonic acids). Other known stripper compositions and processes are described in U.S. Pat. No. 4,424,097 and U.S. Pat. No. 4,687,545. Nitric acid-based strippers also have long been used in the art. See, e.g., the discussion in U.S. Pat. No. 4,713,144, and the use therein of a composition of nitric acid, sulfamic acid and ferric nitrate.
Difficulties arise with all these known stripper compositions. The peroxide-fluoride system is very exothermic and, unless the solution is cooled continuously, temperatures are quickly reached where the peroxide decomposes. Still a further problem with the use of peroxide-fluoride systems in the stripping of lead or tin-lead alloys is the formation of a large volume of lead fluoride sludge which eventually interferes with stripping, requires frequent cleaning of tanks and equipment, and poses significant waste disposal problems.
Strippers based upon nitro-substituted aromatic compounds are prone to redeposition of tin onto the copper from the stripping bath, which can be difficult to remove without excessive attack on the copper, and also are plagued by sludge formation. Nitric acid-based strippers form large volumes of sludge which, apart from the above-noted problems, can become adhered to the surfaces of the copper or to the printed circuit board substrate. Still further, most such systems require two-step processing in order to insure removal of copper-tin intermetallic residing on the copper surface after first removal of tin, lead or tin-lead.
The formation of sludges and the highly corrosive nature of many of these stripper solutions also generally limits their practical use to processes where the surfaces to be treated are dipped or immersed in the solution, i.e., they are incompatible with the equipment which would be used to apply the solutions by a spraying technique.
It is known in the art that the formulation of a tin, lead or tin-lead stripping composition which is capable of economically rapid and complete removal of the tin and/or lead and/or tin-lead and/or tin-copper layers on a copper substrate surface is subject to competing considerations of, on the one hand, sufficient aggressiveness to achieve these purposes while, on the other hand, not so aggressive as to result in any significant attack on the underlying copper substrate itself which could affect its current carrYing capacity, adhesion to or coverage of an underlying non-conductive substrate, and the like. To this end, it is known to include in the compositions agents which inhibit copper attack. For example, in U.S. Pat. Nos. 3,926,699 and U.S. Pat. No. 3,990,982, it is stated that ammonia (e.g., in the form of ammonium bifluoride) serves this function in a hydrofluoric acid-hydrogen peroxide stripping composition; U.S. Pat. No. 4,713,144 states that the sulfamic acid in its nitric acid/ferric nitrate/sulfamic acid composition serves as an inhibitor to prevent nitric acid attack on the copper; U.S. Pat. No. 4,297,257 discloses the addition of polyacrylamide for such purpose to a peroxide-fluoride stripping composition; U.S. Pat. No. 4,306,933 discloses the addition of amino acids and certain aromatic carboxylic acids to a peroxide-fluoride stripping composition; and U.S. Pat. No. 4,374,744 discloses heterocyclic nitrogen compounds (e.g., pyrroles, pyrazoles, imidazoles, triazoles) as additives to stripping compositions based upon an oxidizing agent and an inorganic or organic acid.
The inhibiting of a system to prevent its attack on copper can, of course, at the same time unduly inhibit the tin, lead or tin-lead stripping function itself and/or might lead to problems (or additional problems beyond those presented by the stripping materials themselves) in undesirable by-product of sludge formation, stripped metal redeposition, and the like.